Several types of star-shaped polymers have been prepared recently. The more conventional star polymers have a random molecular structure characterized by lack of symmetry, cross-linking between arms and a large molecular volume. The viscosity of the bulk polymer itself generally is higher than that of any solution thereof. More recently certain highly symmetrical, dense, non-cross-linked star polymers have been described (see for instance U.S. Pat. No. 4,507,466 Mar. 26, 1985 Tomalia et al). In this patent, a regular dendritic star structure was formed by condensation polymerization of an electrophilic monomer with a nucleophilic monomer e.g. dendritic polyamidoamine formed from ammonia, methylacrylate and excess ethylene diamine, with further alternating reaction with the latter two reactants. These dendritic stars are not amenable to the addition of living prepolymerized chains to the end of each arm because the whole dendrite is reactive toward reagents used to create the living end and irregular grafting over the entire structure would occur. A review of this latter type of star polymer is given in Angew. Chem. Int. Ed. Engl. 29 (1990) p. 138-175 Tomalia et al.
Initial work has been done on certain hybrid star polymers in which the starting dendritic structure is an octa-, dodeca-, or decaoctachlorosilane and linear arms are attached at the chloro sites using living prepolymers of isoprene (see Macromolecules 13(1980) p. 191-193 Hadjichristidis et al). Substantially complete reaction at the chloro sites was achieved. See also Macromolecules 22(1989) p. 1897-1903 Roovers et al.
We have continued to investigate this type of hybrid star polymer and have found that an anomaly in solution properties occurs when the chlorosilane core is built up in layers until at least about 48 peripheral active sites for arm linkage is achieved.